This invention relates to a ferroelectric liquid crystal material. More particularly it relates to a ferroelectric liquid crystal composition comprising at least one achiral smectic liquid crystal compound and at least one optically active compound and having high-speed response properties and a superior alignment, and also to a light switching element using the composition.
2. Description of the Related Art
Liquid crystal compounds have been broadly used as materials for liquid crystal display elements, but most of such liquid crystal display elements are those of TN display mode and as liquid crystal materials, those belonging to nematic phase have been used.
Since TN display mode is non-emissive, it has specific features that eyes are not tired and the power consumption is very small, but on the other hand, it has drawbacks that the response is slow and the display is not seen depending on the viewing angle.
The above-mentioned mode has recently been turning toward a direction making use of its characteristic of flat display, and in particular, high-speed response properties and a broad viewing angle have been required.
In order to satisfy such requirements, various improvements in liquid crystal materials have been made. However, as compared with other emissive mode displays (such as electroluminescence display, plasma display, etc.), TN display mode is observed to have a considerably large difference from the above display modes in the aspects of the response time and the breadth of viewing angle.
In order to make use of the specific features of non-emissive mode and small power consumption and also to ensure response properties matching those of emissive mode display, development of a novel liquid crystal display mode in place of TN display mode is indispensable.
As one of such attempts therefor, a display mode utilizing the light switching phenomenon of ferroelectric liquid crystals has been proposed by N. A. Clark and S. T. Lagewall (see Appl. Phys. Lett. 36, 899, 1980).
The presence of ferroelectric liquid crystals was announced by R. B. Meyer et al in the year of 1975 for the first time (see Journal de Physique, 36, 69, 1975), and the crystals belong in the aspect of liquid crystal structure to chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase and chiral smectic H phase (hereinafter abbreviated to SC* phase, SI* phase, SF* phase, SG* phase and SH* phase, respectively).
In the chiral smectic phases, molecules form layers and are tilted against the surfaces of the layers, the helical axis being perpendicular to the surfaces.
In the chiral smectic phases, a spontaneous polarization is formed; hence when a direct current electric field is impressed in parallel to the layers, the molecules are inverted around the helical axis as a rotating axis, depending on the polarity. Display elements using ferroelectric liquid crystals utilize this switching phenomenon.
Among the chiral smectic phases, SC* phase has now been particularly noted.
As to the display mode utilizing the switching phenomenon of SC* phase, the following two modes may be considered:
one mode is a birefringence mode using two polarizers and the other mode is a guest-host mode using a dichroic dyestuff.
Such display mode has the following specific features:
(1) the response time is very rapid; PA1 (2) there are memory properties; PA1 (3) the viewing angle-dependency is small; etc.;
thus the mode has a possibility of a high density display and hence it is very attractive for display elements.
Ferroelectric liquid crystal materials used for ferroelectric liquid crystal display elements for practical use require a number of specific features, but at present, there is no single compound which satisfies these specific features; hence it is necessary to use ferroelectric liquid crystal compositions obtained by blending some liquid crystal compounds or non liquid crystal compounds.
Further, ferroelectric liquid crystal compositions are not limited only to those consisting only of ferroelectric liquid crystal compounds, but also it has been reported in Japanese patent application laid-open No. Sho 61-195187/1986 that when at least one member of compounds exhibiting ferroelectric liquid crystal phase is blended with compound(s) or composition(s) exhibiting achiral smectic C, F, G, H, I phase or the like (hereinafter abbreviated to SC phase or the like) as basic substance(s), it is possible to make the whole a ferroelectric liquid crystal composition. Further, it has also been reported that when at least one member of compounds which are optically active but exhibit no ferroelectric liquid crystal phase is blended with compound(s) or composition(s) exhibiting SC phase or the like as base substance(s), it is possible to make the whole a ferroelectric liquid crystal composition (Mol. Cryst. Liq. Cryst., 89, 327 (1982)).
In summary of these facts, it is seen that when 10 at least one member of optically active compounds, irrespective of whether or not the compound exhibits ferroelectric liquid crystal phase, is blended with basic substance(s), it is possible to form a ferroelectric liquid crystal composition.
At present, a number of specific features have been required for ferroelectric liquid crystal materials, but for example when they are used for practical display elements (640 lines.times.400 lines), the following facts are particularly required:
(1) SC* phase is exhibited within a broad temperature range including room temperature (at least 0.degree. C. to 50.degree. C.);
(2) the response time is 100 .mu.sec or less; and
(3) the alignment is superior. At present, however, no ferroelectric liquid material (liquid crystal composition) satisfying all of such conditions has yet been obtained.
For example, official gazette of Japanese patent application laid-open No. Sho 61-291679/1986 and pamphlet of PCT international application laid-open No. W086/06401 disclose ferroelectric liquid crystal mixture(s) obtained by blending an achiral 5-alkyl-2-(4-alkoxyphenyl)pyrimidine having SC phase with optically active compound(s), which mixture(s) exhibit SC* phase within a broad temperature range including room temperature. Further, the former gazette discloses that since the pyrimidine derivative has a very low viscosity, its use in the form of a base SC mixture is very effective for improving the response properties of ferroelectric compositions. However, the ferroelectric liquid crystal composition(s) satisfy the above requirement as regards the temperature range of SC* phase, but the response time is 300 .mu.sec to 500 .mu.sec (e.g. see ferroelectric liquid crystal compositions described in Examples 1 and 2 of the above gazette and those described in Examples 45 and 46 of the pamphlet); hence the composition(s) are difficultly regarded as practical.
Further, the specification of Japanese patent application No. Sho 62-137883/1987 discloses ferroelectric liquid crystal composition(s) obtained by blending an achiral 5-alkoxy-2-(4-alkylphenyl)pyrimidine having SC phase with optically active compound(s), which composition(s) exhibit SC* phase within a broad temperature range including room temperature and yet have very high-speed response properties. For example, the ferroelectric liquid crystal composition described in Example 5 exhibits SC* phase within a broad temperature range of 5.degree. to 52.degree. C. and the response time is 50 .mu.sec; hence the composition satisfies the above requirements (1) and (2). Thus, the composition is considered to be very practical as compared with the ferroelectric liquid crystal compositions described in the above gazette and pamphlet.
However, the ferroelectric liquid crystal composition(s) described in the above specification of Japanese patent application No. Sho 62-137883/1987 have a serious problem when they are practically used. Namely, the composition(s) have no cholesteric phase; hence according to alignment technique currently employed for TN liquid crystal materials, no uniform alignment cannot have been achieved.
At present, as the alignment for ferroelectric liquid crystal materials, three methods of shearing method, temperature gradient method and surface treatment method have been attempted. According to the shearing method, alignment is effected by applying a shearing stress in smectic A phase, and the temperature gradient method is a method similar to epitaxial crystal growth method, having noted the fact that smectic phase can be regarded as one-dimensional crystal.
The surface treatment method has been practically employed for aligning TN liquid crystal materials and is a method of coating the substrate of a cell with a high-molecular film such as polyimide film, followed by subjecting the resulting surface to rubbing treatment to align liquid crystal molecules. From the viewpoint of commercial production of liquid crystal display elements, it is most preferred to align liquid crystal molecules according to the surface treatment method.
The phase transition forms of ferroelectric liquid crystal materials include the following four: (i) Iso phase.fwdarw.SC* phase, (ii) Iso phase.fwdarw.N* phase.fwdarw.SC* phase, (iii) Iso phase.fwdarw.SA phase.fwdarw.SC* phase and (iv) Iso phase.fwdarw.N* phase.fwdarw.SA phase.fwdarw.SC* phase wherein Iso phase, N* phase and SA phase represent isotropic liquid phase, cholesteric phase and smectic A phase, respectively.
Among these, ferroelectric liquid crystal materials for which the current alignment technique (surface treatment method) is employable as it is, are those having the phase transition form expressed by (iv) (e.g. see Japanese patent application laid-open No. Sho 6-250086/1986). Thus, ferroelectric liquid crystal materials having the phase transition form expressed by (iv) have been earnestly desired.
The ferroelectric liquid crystal composition(s) disclosed in the above Japanese patent application No. Sho 62-137883/1987 have no cholesteric phase; hence in order to uniformly align these compositions, the shearing method or the temperature gradient method should be employed, and a long time is required for the alignment so that the alignment cannot be easily effected. Further, since the current alignment technique cannot be employed as it is, investment for a new equipment is required; hence the above ferroelectric liquid crystal composition(s) are difficultly regarded as practical.
As apparent from the foregoing, currently known ferroelectric liquid crystal materials cannot be yet regarded as practical; hence a further improvement in the specific features thereof has been earnestly desired.
The present inventors have made extensive research in order to further improve the invention described in the above Japanese patent application No. Sho 62-137883/ 1987. As a result, we have found that when compounds are combined together as shown below, there is obtained a ferroelectric liquid crystal composition exhibiting SC* phase within a broad temperature range including room temperature and yet having cholesteric phase and further having high-speed response properties, and have achieved the present invention.